Criteria to Determine Borehole Formation Temperatures for Calibration of Basin and Petroleum System Models
Kenneth E. Peters1 and Philip A. Nelson2
1IES-Schlumberger, Aachen, Germany and Houston, Texas
2U.S. Geological Survey, Denver, Colorado
Bottom-hole temperatures (BHT) from well log headers are common, but require correction because they are biased lower than true formation temperatures. Care must be taken to avoid short static times, re-circulation between measurements, and spurious records of times or temperatures from logs. Criteria for reliable Horner corrected BHT data include a minimum of three logging runs that record time and temperature for each run, temperature extrapolation less than the range of temperature data, and deviations from the least square regression line that are less than measurement uncertainty (±1-3°C or ±2-5°F).
BHT data are an important source of uncertainty that needs to be considered when calibrating basin and petroleum system models. Based on published comparisons of DST and Horner-corrected BHT data from the same depths (e.g., Hermanrud et al., 1990), the standard deviation of corrected bottom-hole temperatures is about ±8°C (±14°F).
A Petromod® 1D burial history for a pseudowell in the upper Cook Inlet serves as an example of the effect of BHT error on prediction of the thermal maturity of deep source rock. A single BHT of 92.4°C results in top oil window for the Jurassic Tuxedni Group source rock at 82.50 Ma and 2,409 m depth (Figure 1). BHT values 8°C above or below 92.4°C (one standard deviation) result in top oil window 6.2 my (7.6%) earlier and 305 m (12.7%) shallower or 4.5 my (5.5%) later and 231 m (9.6%) deeper than that calculated using the 92.4°C BHT, respectively. Likewise, a BHT of 92.4°C results in top gas window for the Tuxedni Group source rock at 22.73 Ma and 4,187 m depth. BHT values 8°C above or below 92.4°C result in top gas window 1.8 my (7.7%) earlier and 312 m (7.5%) shallower or 1.6 my (7.1%) later and 249 m (5.9%) deeper than that calculated using the 92.4°C BHT, respectively.
Figure 2 shows the range of age to the top of the oil (0.6% vitrinite reflectance; Ro) and gas windows (1.2% Ro) based on the plausible error in BHT (shaded areas). The best fit scenario (bold vertical lines) results in an interpretation where most petroleum from the Tuxedni Group source rock was lost due to generation and migration prior to formation of the reservoir rock (Hemlock Conglomerate; stippled) and seal rock (lower part of the Tyonek Formation; horizontal lines). In this simple case, the interpretation from the model is not seriously affected by the error associated with the BHT measurement.
An older model (Magoon, 1994) required that petroleum generated from the Tuxedni Group source rock in Paleocene time migrate updip to the basin flanks where it was trapped stratigraphically until structural traps formed during Miocene deformation. Unlike the older model, our new model suggests that petroleum generation began in Upper Cretaceous rather than Paleocene time. However, both models confirm that the generation-migration-accumulation process continues today and that petroleum generated prior to deposition of the Hemlock Conglomerate reservoir rock (~26-32 Ma) and the overlying seal rock in the lower portion of the Tyonek Formation seal rock (~20-26 Ma) could be lost. Both models suggest that only a highly mature fraction of the generated petroleum consisting mainly of gas with some condensate could be trapped because of the relative timing of generation and formation of reservoir and seal rock.
Hermanrud, C., Cao, S., and Lerche, I., 1990, Estimates of virgin rock temperature derived from BHT measurements: bias and error. Geophysics 55, 924-931.
Magoon, L.B., 1994, Tuxedni-Hemlock(!) petroleum system in Cook Inlet, Alaska, U.S.A., in Magoon, L.B., and Dow, W.G., eds., The Petroleum System - From Source to Trap. American Association of Petroleum Geologists Memoir 60, p. 359-370.
Sweeney, J.J., and Burnham, A.K., 1990, Evaluation of a simple model of vitrinite reflectance based on chemical kinetics: American Association of Petroleum Geologists Bulletin, v. 74, p. 1559-1570.
Figure 1. Influence of BHT error (±8oC or 14oF) on calculated age (my, top) and burial depth (ft, lower) of the Jurassic Tuxedni Group source rock at 0.6% vitrinite reflectance (Ro, %; top of oil window) in a pseudowell from the upper Cook Inlet, Alaska. The ten data points are sample error calculations. Open dot is the best fit scenario where Petromod® calibration to a BHT of 92.4oC results in calculated top oil window for the source rock at 82.50 Ma and 2,409 m depth.
Figure 2. Calculated vitrinite reflectance (Ro, %; dashed line) for Jurassic Tuxedni Group source rock in a pseudowell from the upper Cook Inlet, Alaska based on a Petromod® 1D model that used the EASY%Ro method of Sweeney and Burnham (1990). Top of oil and gas windows occurred at 0.6% and 1.2% Ro (solid vertical lines at 82.5 and 22.7 Ma, respectively). Shaded areas define uncertainty of the calibration if one assumes a BHT of 92.4oC ±8oC (198.3 ±14oF). Reservoir rock (Hemlock Conglomerate; stippled) and seal rock (lower portion of Tyonek Formation; horizontal lines) were deposited approximately 32-26 Ma and 26-20 Ma, respectively (Magoon, 1994), thus allowing accumulation of late-generated petroleum from the source rock.
AAPG Search and Discovery Article #90091©2009 AAPG Hedberg Research Conference, May 3-7, 2009 - Napa, California, U.S.A.